To meet the recent demand for higher integration in integrated circuits, pattern formation to a finer feature size is required. Acid-catalyzed chemically amplified resist compositions are most often used in forming resist patterns with a feature size of 0.2 μm or less. High-energy radiation such as UV, deep-UV or EB is used as the light source for exposure of these resist compositions. In particular, while EB lithography is utilized as the ultra-fine microfabrication technique, it is also indispensable in processing a photomask blank to form a photomask for use in semiconductor device fabrication.
Polymers comprising a major proportion of aromatic structure having an acidic side chain, for example, polyhydroxystyrene have been widely used in resist materials for the KrF excimer laser lithography. These polymers are not used in resist materials for the ArF excimer laser lithography since they exhibit strong absorption at a wavelength of around 200 nm. These polymers, however, are expected to form useful resist materials for the EB and EUV lithography for forming patterns of finer size than the processing limit of ArF excimer laser because they offer high etching resistance.
Resist compositions for photolithography include positive ones in which exposed areas are dissolved away and negative ones in which exposed areas are left as a pattern. A suitable composition is selected among them depending on the desired resist pattern. In general, the chemically amplified negative resist composition comprises a polymer which is normally soluble in an aqueous alkaline developer, an acid generator which is decomposed to generate an acid upon exposure to light, and a crosslinker which causes the polymer to crosslink in the presence of the acid serving as a catalyst, thus rendering the polymer insoluble in the developer (sometimes, the crosslinker is incorporated in the polymer). Typically a basic compound is added for controlling the diffusion of the acid generated upon light exposure.
A number of negative resist compositions of the type comprising a polymer which is soluble in an aqueous alkaline developer and includes phenolic units as the alkali-soluble units were developed, especially as adapted for exposure to KrF excimer laser light. These compositions have not been used in the ArF excimer laser lithography because the phenolic units are not transmissive to exposure light having a wavelength of 150 to 220 nm. Recently, these compositions are recognized attractive again as the negative resist composition for the short wavelength (e.g., EB or EUV) lithography capable of forming finer size patterns. Exemplary compositions are described in JP-A 2006-201532, JP-A 2006-215180, and JP-A 2008-249762.
Improvements were made in the control of resist sensitivity and pattern profile by properly selecting and combining components used in resist compositions and adjusting processing conditions. One outstanding problem is the diffusion of acid that has a material impact on the resolution of a chemically amplified resist composition.
An acid diffusion regulator is, in fact, essential for controlling acid diffusion and improving the performance, especially resolution of a resist composition. Studies have been made on the acid diffusion regulator while amines and weak acid onium salts have been generally used. The weak acid onium salts are exemplified in several patent documents. JP 3955384 describes that the addition of triphenylsulfonium acetate ensures to form a satisfactory resist pattern without T-top profile, a difference in line width between isolated and grouped patterns, and standing waves. JP-A H11-327143 reports improvements in sensitivity, resolution and exposure margin by the addition of sulfonic acid ammonium salts or carboxylic acid ammonium salts. Also, JP 4231622 describes that a resist composition for KrF or EB lithography comprising a PAG capable of generating a fluorinated carboxylic acid is improved in resolution and process latitude such as exposure margin and depth of focus. Further, JP 4116340 describes that a resist composition for F2 laser lithography comprising a PAG capable of generating a fluorinated carboxylic acid is improved in line edge roughness (LER) and solves the footing problem. While these four patent documents refer to the KrF, EB and F2 lithography, JP 4226803 describes a positive photosensitive composition for ArF excimer laser lithography comprising a carboxylic acid onium salt. These systems are based on the mechanism that a salt exchange occurs between a weak acid onium salt and a strong acid (sulfonic acid) generated by another PAG upon exposure, to form a weak acid and a strong acid onium salt. That is, the strong acid (sulfonic acid) having high acidity is replaced by a weak acid (carboxylic acid), thereby suppressing acid-catalyzed decomposition reaction of acid labile group and reducing or controlling the distance of acid diffusion. The onium salt apparently functions as an acid diffusion regulator.
However, when a resist composition comprising the foregoing carboxylic acid onium salt or fluorocarboxylic acid onium salt is used in patterning, a problem of LER arises. It would be desirable to have an acid diffusion regulator capable of minimizing LER.